Obesity is associated with an increase in the overall amount of adipose tissue (i.e., body fat), especially adipose tissue localized in the abdominal area. Obesity has reached epidemic proportions in the United States. The prevalence of obesity has steadily increased over the years among all racial and ethnic groups. The most recent data from the Centers for Disease Control and Prevention, and the National Center for Health Statistics report 66% of the adult population overweight (BMI, 25.0-29.9), 31% obese (BMI, 30-39.9), and 5% extremely obese (BMI, ≧40.0). Among children aged 6 through 19 years, 32% were overweight and 17% were obese. This translates to 124 million Americans medically overweight, and 44 million of these deemed obese. Obesity is responsible for more than 300,000 deaths annually, and will soon overtake tobacco usage as the primary cause of preventable death in the United States. Obesity is a chronic disease that contributes directly to numerous dangerous co-morbidities, including type 2 diabetes, cardiovascular disease, inflammatory diseases, premature aging, and some forms of cancer. Type 2 diabetes, a serious and life-threatening disorder with growing prevalence in both adult and childhood populations, is currently the 7th leading cause of death in the United States. Since more than 80% of patients with type 2 diabetes are overweight, obesity is the greatest risk factor for developing type 2 diabetes. Increasing clinical evidence indicates that the best way to control type 2 diabetes is to reduce weight.
The most popular over-the counter drugs for the treatment of obesity, phenylpropanolamine and ephedrine, and the most popular prescription drug, fenfluramine, were removed from the marketplace as a result of safety concerns. Drugs currently approved for the long-term treatment of obesity fall into two categories: (a) CNS appetite suppressants such as sibutramine and (b) gut lipase inhibitors such as orlistat. CNS appetite suppressants reduce eating behavior through activation of the ‘satiety center’ in the brain and/or by inhibition of the ‘hunger center’ in the brain. Gut lipase inhibitors reduce the absorption of dietary fat from the gastrointestinal (GI) tract. Although sibutramine and orlistat work through very different mechanisms, they share in common the same overall goal of reducing body weight secondary to reducing the amount of calories that reach the systemic circulation. Unfortunately, these indirect therapies produce only a modest initial weight loss (approximately 5% compared to placebo) that is usually not maintained. After one or two years of treatment, most patients return to or exceed their starting weight. In addition, most approved anti-obesity therapeutics produce undesirable and often dangerous side effects that can complicate treatment and interfere with a patient's quality of life.
The lack of therapeutic effectiveness, coupled with the spiraling obesity epidemic, positions the ‘treatment of obesity’ as one of the largest and most urgent unmet medical needs. There is, therefore, a real and continuing need for the development of improved medications that treat or prevent obesity.
The endocanabinoid system, comprised of the canabinoid receptors (CB1 and CB2) and their endogenous ligands (e.g., anandamide, 2-AG), plays a prominent role in the control of food intake and energy metabolism. CB1 receptors are widely expressed in the brain, including cortex, hippocampus, amygdala, pituitary and hypothalamus. CB1 receptors have also been identified in numerous peripheral organs and tissues, including thyroid gland, adrenal gland, reproductive organs, adipose tissue, liver, muscle, and gastrointestinal tract. CB2 receptors are localized almost exclusively in immune and blood cells [Endocrine Reviews 2006, 27, 73].
The plant-derived cannabinoid agonist Δ9-tetrahydrocannabinol (Δ9-THC), the main psychoactive component of marijuana, binds to both CB1 and CB2 receptors. Δ9-THC is widely reported to increase appetite and food intake (hyperphagia) in humans and in animals. This hyperphagic effect is largely blocked by pretreatment with selective CB1 receptor antagonists/inverse agonists [e.g., rimonabant (SR141716A, Acomplia®)], strongly supporting the belief that CB1 receptor activation mediates the hyperphagic effect of Δ9-THC, [Endocrine Reviews 2006, 27, 73].
In humans, rimonabant produces a clinically meaningful weight loss in obese patients. Patients also experience improvements in associated cardiometabolic risk factors, including a decrease in fasting insulin levels and a decrease in triglyceride levels. Rimonabant also produces greater reductions in abdominal fat deposits, which are a known risk factor for diabetes and heart disease [Science 2006, 311, 323]. Taken together, these improvements in adiposity and cardiometabolic risk factors produce an overall decrease in the prevalence of the metabolic syndrome [Lancet 2005, 365, 1389 and NEJM 2005, 353, 2121].
The beneficial effects of rimonabant on cardiometabolic risk factors such as high blood pressure, insulin resistance and eleveated levels of blood lipids cannot be explained by diet and weight loss alone. For example, in patients receiving 20 mg of rimonabant, only approximately 50% of the beneficial effects on triglycerides, fasting insulin, and insulin resistance can be accounted for by weight loss. These results suggest a direct pharmacological effect of CB1 antagonists on glucose and lipid metabolism, in addition to indirect effects on metabolism secondary to hypophagia-mediated weight loss [Science 2006, 311, 323 and JAMA 2006, 311, 323].
The CB1 receptor is one of the most abundant and widely distributed G protein-coupled receptors in the mammalian brain. It is believed that the appetite-suppressant properties of CB1 antagonists are mediated through an interaction with CB1 receptors in the hypothalamus (regulation of food intake), and in the mesolimbic region (rewarding properties of food). However, CB1 receptors are far more broadly distributed in brain (e.g., neocortex, hippocampus, thalamus, cerebellum, and pituitary), and while interacting with targeted CB1 receptors in hypothalamus and mesolimbic regions, CB1 antagonists have ready access to non-targeted CB1 receptors that have little if any role in appetite control. Binding to non-targeted receptors can often lead to unwanted side effects of CNS drugs [Endocrine Reviews 2006, 27: 73]. The CB1 antagonist/inverse agonist rimonabant produces psychiatric and nervous system side effects. These include depressed mood, anxiety, irritability, insomnia, dizziness, and headache. These side effects are dose-related and are most pronounced at the most efficacious weight-reducing dose of rimonabant (JAMA 2006, 311, 323). The occurrence of therapeutic efficacy (appetite suppression) and side effects over the same dose range strongly suggest that both effects are mediated through concurrent antagonism of CB1 receptors in both ‘targeted’ and ‘non-targeted’ brain regions. Brain-penetrant CB1 antagonists/inverse agonists do not selectively target CB1 receptors in efficacy brain regions, while ignoring CB1 receptors in side effect brain regions.
The beneficial effects of the CB1 antagonist rimonabant on body weight, adiposity, and cardiometabolic risk factors such as high blood pressure, insulin resistance and elevated levels of blood lipids cannot be explained by weight loss derived from CNS-mediated appetite suppression alone [JAMA 2006, 311, 323]. Approximately 50% of the non-CNS benefit is likely derived from an interaction with CB1 receptors in peripheral tissues known to play an active role in metabolism. These include adipose tissue, liver, muscle, and gastrointestinal tract.
In view of the above, it is highly desirable to find effective and highly selective CB1 receptor antagonists/inverse agonists with limited or no CNS adverse side effects, including mood disorders. Particularly, it is desirable to find compounds that preferentially target CB1 receptors in peripheral tissues (e.g., adipose tissue, liver, muscle, and gastrointestinal tract), while sparing CB1 receptors in brain. In this way, peripherally-mediated beneficial effects of CB1 antagonists/inverse agonists should be maintained, whereas CNS side effects should be reduced or eliminated. This should provide a novel opportunity to develop safer agents for the prevention or treatment of obesity, diabetes, and cardiometabolic diseases (e.g., hypertension and dyslipidemias).